1. Field of the Invention
This invention relates to a process for determining the charge carrying activity, in generally non-conductive, materials such as paint, dental resin, B-staged resin, concrete, dielectric fluids, food, etc. More particularly, the present invention pertains to the detection of changes in charge carrying activity of nearly any non-conductive/semi-liquid material, so as to enable a user to determine the extent of chemical change, and thus the amount of curing (paints, adhesives and long chain polymers), remaining useful life (dielectric fluids) or other conditions of a given material during times other than rapid state transitions.
2. Prior Art
The detection of cross-linking in low-conductive polymeric materials became common place in the thermosetting resin industry to determine when thermosetting resins are properly cured. These resins form a class of very useful plastics which have been applied throughout the aerospace industry, construction industry, automotive manufacturing, medical applications, adhesives, and in virtually every area where permanent characteristics of weatherability, structural stiffness, strength and ease of manufacture through molding process provides an advantage over competing metals, ceramics and other compositions. Dental applications include filling and facia materials which are applied to the tooth in liquid form and then polymerized by UV radiation or other known techniques. Many paint compositions are a form of thermosetting resin whose application depends on having a uniform liquid state which can be readily applied by brush or air gun. Matched die, filament winding, transfer molding, lay up molding and pultrusion techniques for fabricating structural and component parts, housings, etc., depend on maintenance of a flowable condition which can wet fibers or quickly fill mold cavities in a liquid state.
Tests for determining cross-linking within the resins were developed to test when the resins had properly cured, i.e. passed through a state transition from a flowable resin to a thermoset solid. These resin materials are typically manufactured in a low viscous liquid state wherein the polymer material has incurred minimal cross-linking prior to the curing stage. It is, of course, this cross-linking that causes the state transition by solidifying the thermosetting composition into a permanent, rigid structure characterizing this group of plastics. The cross-linking of the polymers typically occurs under high heat, which is commonly referred to as the curing stage.
There is increasing interest in the composites industry to monitor, adjust and optimize the cure cycle of thermoset polymers. Accordingly, it is known to evaluate cross-linking during actual cure using viscometers, infrared meters, and microdielectrometers. This period of evaluation is characterized by the resins being subjected to high temperatures used to fully complete the curing of the materials. The primary interest in rapid state transition is to identify the gelation point and then to confirm final stage at which the curing process is complete, so that the final product can be removed without extending cure time and conditions beyond that which is necessary. This enables efficient use of expensive equipment and also insures that the manufactured part is not removed from the mold prior to complete cross-linking.
While measuring the cross-linking within the thermosetting resin is important during the curing stage, it is also important to determine the amount of cross-linking which has occurred in the pre-cure stage, i.e. the time between manufacture of the resin and the time at which the resin is cured. During this period which is often referred to as the shelf life of the product, the resin under goes a very slow transition from one state to another via the cross-linking of polymers. However, until the present invention, those skilled in the art believed that the measurement of generally nonconductive liquids and semi-liquids (i.e. during the shelf life or slow state transition stage) if possible, would be impractical and expensive at the ambient or low temperatures at which the liquids are maintained to prevent cross-linking.
The amount of curing which occurs while the product remains unused is important to know because premature curing results in a permanent, irreversible condition which makes the material useless for further processing. Indeed, the extent of waste arising because of premature curing of thermosetting materials is substantial. In industries where partially cured materials must be discarded for safety reasons, the losses are even more significant. For example, the manufacture of high performance aircraft components from resins that have already partly cured could result in weakened structures that put lives in jeopardy. Therefore, it is very likely that a substantial amount of good resin is discarded because of suspicion of excessive pre-cure.
Because most resins will inherently begin cross-linking upon manufacture and will continue such cross-linking until finally cured, measures are taken to reduce and control this process. The primary control measure is to maintain the resins at low temperatures to reduce reaction rates to a minimum. This low temperature environment needs to be maintained until the material is ready for final curing. Unfortunately, the resin material appearance does not always reflect the degree of curing which has occurred during this slow state transition (pre-cure) stage. If variations in temperature occur during storage, their impact may be substantially unknown. The low temperatures at which the resins, etc, are kept have also been a major factor in discouraging the use of cross-linking analysis to determine the extent of slow state transition, because most of those skilled in the art believed that a low temperature resin liquid would not provide sufficient indicators of cross-linking, or that the indicators would be hidden in background noise.
With paints and adhesives, viscosity provides a useful measure of acceptability of slow state transition (pre-cure) cross-linking. In general, their shelf life is determined by the time required for the material to set up or become too viscous to flow well. There are, however, no current tests to determine the actual state of cross-linking in paints and adhesives. Current practice is to examine the viscosity of the materials qualitatively as noted, or perform sample tests to determine the performance of these resins in a particular application.
With respect to polymers used in a matrix material for fiber reinforced composites, there are two distinct time periods during which cross-linking takes place. The first period can be called the shelf life of the material and the second is the curing cycle. Users of fiber reinforced thermosetting composites have created several mechanical tests to evaluate the state of cumulative cross-linking in the slow state transition (pre-cure) stages. For example, tack and drape properties give an indication of the extent of cure. These tests are acknowledged to be highly subjective and unreliable, and are at best general qualitative indicators having little quantitative value.
A more specific application of thermosetting resins for composite materials is to impregnate a layer of fiber reinforcement with resin, and then store this "pre-preg" or "B-staged" material for later use. Obviously, this B-staged material will have a limited shelf life, depending upon the rate of continued cross-linking, which is affected mainly by temperature. It is presently difficult, subjective, and consumptive of material to test the B-staged material for the extent of cross-linking. If the B-staged material has reached a particular stage of cross-linkage, it is no longer usable material and must be discarded on the basis of storage time, rather than on the actual amount of cross linking.
Providing a device and method which enables users to derive information about cross-linking during the slow state transition (shelf life) allows repeated measurement of resins, adhesive, paints and the like, to minimize waste and product failure.
Thus, there is needed an effective device and method for measuring low-conductivity or nonconductive materials repeatedly to determine the extent of cross-polymerization during the shelf-life or slow state transition period, etc., to enable more effective determination of the conditions of materials. Such a device and method could provide quantitative determination of which materials should be discarded and which can be safely or effectively used for their intended purpose.